This invention is directed towards the deposition of small (usually fractional gram) masses on a generally electrically non-conductive substrate. One of the most common methods for accomplishing the goal is practiced by manufacturers of photocopiers and electrophotographic electronic printers. This involves causing charged toner particles to migrate with an electric field to a charged area on a photoreceptor, so-called electrostatic deposition. While electrostatic deposition has been proposed for packaging powdered drugs (see U.S. Pat. Nos. 5,669,973 and 5,714,007 to Pletcher), electrostatic deposition is limited by the amount of mass that can be deposited in a given area.
This limitation is intrinsic to electrostatic deposition technology and is determined by the combination of the amount of charge that can be placed on the photoreceptor and the charge to mass ratio of the toner particles. The mass that can be deposited in an area of a substrate is limited to the charge in the area divided by the charge to mass ratio of the particles being deposited. The maximum amount of charge that can be deposited in an area of a substrate is determined by the substrate electrical properties, the electrical and breakdown properties of the air or gas over it, and by the properties of mechanism used for charging the substrate. Likewise, the minimum charge to mass ratio of particles (which determines the maximum mass that can be deposited) is determined by the charging mechanism. However, as the charge to mass ratio is decreased, the variation in the charge to mass ratio increases even to the point where some particles may be oppositely charged relative to the desired charge on the particles. This variation prevents the reliable deposition of a controlled mass on the substrate. Furthermore, low charge to mass ratio particles limit the overall speed of deposition because the force of a particle, which sets the particle velocity, from an electrostatic field is proportional to the charge carried by the particle. For these reasons, higher charge to mass ratio particles are generally preferred.
Packaged pharmaceutical doses, in the range of 15 to 6000 μg are employed in dry powder inhalers for pulmonary drug delivery. A mean particle diameter of between 0.5 and 6.0 μm is necessary to provide effective deposition within the lung. It is important that the dose be metered to an accuracy of +/−5%. A production volume of several hundred thousand per hour is required to minimize production costs. High speed weighing machines are generally limited to dose sizes over about 5,000 μg and thus require the active pharmaceutical be diluted with an excipient, such as lactose powder, to increase the total measured mass. This approach is subject to limitations in mixing uniformity and the aspiration of extraneous matter. Hence, electrostatic deposition of such pharmaceutical powders is highly desirable.
U.S. Pat. No. 3,997,323, issued to Pressman et al, describes an apparatus for electrostatic printing comprising a corona and electrode ion source, an aerosolized liquid ink particles that are charged by the ions from the ion source, a multi-layered aperture interposed between the ion source and the aerosolized ink for modulating the flow of ions (and hence the charge of the ink particles) according to the pattern to be printed. The charged ink particles are accelerated in the direction of the print receiving medium. This patent discusses the advantages in the usage of liquid ink particles as opposed to dry powder particles in the aerosol. However, from this discussion it is apparent, aside from the disadvantages, that dry powder particles may also be used. Furthermore, the charge to mass ratios achieved from using an ion source for charging the powder particles are much higher than those generally achieved using triboelectric charging (commonly used in photocopies and detailed by Pletcher et al in U.S. Pat. No. 5,714,007), thereby overcoming the speed issue discussed above. Such printers have been commercially marketed and sold. However, an apparatus for depositing powder on a dielectric (i.e. a powder carrying package) using the Pressman approach also suffers from the above described maximum amount of powder that can be deposited on the dielectric. This is because during the deposition process, charge from both the ions and the charged particles accumulates on the dielectric, ultimately resulting in an electric field that prevents any further deposition. In other words, the amount of material that can be deposited on the dielectric packaging material is limited by the amount of charge that can be displaced across it which is determined by the capacitance of the dielectric and the maximum voltage that can be developed across it.